Ash trees, prevalent across North American landscapes, are facing a significant threat. These trees, common in urban and forest ecosystems, are experiencing a widespread decline. The concern over their future stems from significant biological challenges that have rapidly accelerated their mortality rates. This situation has led to questions about whether these species can endure the pressures currently impacting them.
The Emerald Ash Borer Invasion
The primary cause of the widespread decline in ash tree populations is the emerald ash borer (EAB), an invasive beetle native to Asia. This metallic green beetle was first detected in North America near Detroit, Michigan, and Windsor, Ontario, in 2002, though it is believed to have arrived in the early to mid-1990s through wood packaging materials. The EAB poses a severe threat to nearly all North American ash species, which have not evolved defenses against it.
Adult EAB beetles lay their eggs in bark crevices of ash trees. Upon hatching, the larvae burrow into the tree, feeding on the inner bark and phloem, which are the tissues responsible for transporting water and nutrients. This feeding creates winding, S-shaped galleries that effectively girdle the tree, disrupting its ability to move essential resources and ultimately leading to its death. The life cycle usually takes one year, though it can extend to two years in cooler climates or less vigorous trees.
The impact of EAB is rapid; up to 99% of ash trees can be killed within 8 to 10 years of the beetle’s arrival in an area. Since its discovery, EAB has spread to over 37 states and five Canadian provinces, killing millions of ash trees in forests and urban environments. This rapid spread is facilitated by the beetle’s ability to fly locally and, more significantly, by human transportation of infested firewood and nursery stock over long distances.
Other Threats to Ash Trees
While the emerald ash borer is the most prominent threat, ash trees also contend with other diseases and environmental stressors. Fungal diseases are a common issue, including ash anthracnose, which causes brown or black spots on leaves, and verticillium wilt, a soil-borne fungus that impacts the tree’s vascular system, leading to wilting and dieback. Ash yellows, caused by a bacterial parasite called phytoplasma, also affects the tree’s vascular system, leading to slow growth, thin leaves, and eventual branch dieback; there is currently no effective treatment for ash yellows.
Beyond pests and diseases, environmental factors like habitat loss and climate change further compromise ash tree health. Climate change can alter regional climates, potentially influencing the spread and impact of pests like EAB and reducing the resilience of ash trees to infestation. Extreme weather events and altered precipitation patterns can still stress trees, making them more vulnerable to attack by insects and pathogens.
Consequences of Ash Tree Loss
The widespread mortality of ash trees due to the emerald ash borer and other factors has significant ecological and economic consequences. Ecologically, ash trees are a foundational species in many North American forests, supporting a diverse array of wildlife. Their loss directly impacts numerous species, including nearly 300 arthropod species, such as butterflies, moths, beetles, and flies, that rely on ash trees for food and habitat. The disappearance of ash trees also removes a food source for birds and small mammals that consume their seeds.
Ash tree removal creates significant canopy gaps in forests, which can lead to increased light availability on the forest floor. This change can promote the growth and spread of invasive plant species, further disrupting native ecosystems and altering forest composition. The loss of ash also affects nutrient cycling and forest succession, potentially making habitats more vulnerable to other disturbances.
Economically, the costs associated with ash tree loss are substantial. Municipalities face significant expenses for removing dead trees from urban areas. The estimated value of ash timber in U.S. forests is billions of dollars, with significant annual losses from the timber industry. Beyond the timber industry, the loss of shade trees in urban settings can impact property values and increase energy consumption due to reduced cooling effects.
Conservation Efforts for Ash Trees
Strategies to combat ash tree decline include targeted pesticide treatments for high-value ash trees, particularly in urban and residential areas. Systemic insecticides like emamectin benzoate can protect individual trees from EAB infestation, though these treatments typically need to be reapplied every two to three years.
Biological control methods use EAB’s natural enemies from Asia. Four tiny, stingless wasp species, such as Tetrastichus planipennisi and Oobius agrili, have been released in the United States. These parasitic wasps target EAB larvae or eggs, helping to reduce beetle populations, with some studies showing high parasitism rates. While biological control agents alone will not eradicate EAB, they are a long-term, sustainable strategy to balance EAB populations.
Research into breeding EAB-resistant ash varieties is a key conservation effort. Scientists are identifying “lingering ash” trees that have survived in EAB-infested areas, indicating natural resistance. These resistant trees are then used in breeding programs, sometimes by crossing them with Asian ash species that naturally tolerate EAB, to develop genetically diverse populations of resistant North American ash. Replanting programs also emphasize using a diverse mix of tree species, including EAB-resistant ash if available, to enhance the resilience of forests against future threats.
The Future of Ash Trees
The question of whether ash trees are going extinct is complex. While the emerald ash borer has caused widespread mortality, leading to the listing of several North American ash species as critically endangered, complete extinction across the continent is not the most likely outcome. Many experts anticipate the “functional extirpation” of the ash genus, meaning they may no longer fulfill their traditional ecological or economic roles as mature trees in many areas. This implies a significant reduction in their abundance and size, rather than total disappearance.
Despite the impact, there is hope for the long-term persistence of ash trees. Some individual ash trees exhibit natural EAB resistance, and these “lingering ash” are crucial for breeding programs aimed at developing more resilient populations. While ash seedlings and saplings can regenerate abundantly in infested areas, they often succumb to EAB before reaching maturity or producing seeds, indicating that active beetle populations persist.
The ongoing conservation efforts, including biological controls and the development of resistant ash varieties, aim to reduce EAB populations to levels that allow ash trees to survive and reproduce more successfully. However, the recovery of ash trees to their former prominence will be a very long process, potentially centuries, as growing mature ash is slow. Continued monitoring, research, and public involvement in preventing the spread of EAB remain important for shaping the future of ash trees in North America. The goal is for ash to persist, even if in a diminished capacity initially, with the potential for eventual recovery through natural selection and human intervention.